A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed and put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, e.g., zinc oxide, and a resin binder is subjected to an ordinary electrophotographic processing to form a highly lipophilic toner image thereon and the surface of the photoreceptor is then treated with an oil-desensitizing solution, often referred to as an etching solution, to selectively render non-image areas hydrophilic, to thereby obtain an offset printing plate.
Requirements of offset printing plate precursors for obtaining satisfactory prints are such that: an original should be reproduced faithfully on the photoreceptor; the surface of a photoreceptor has affinity with an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic, while, at the same time, having water resistance; and that a photoconductive layer having an image formed thereon is not released during printing and is well receptive to dampening water so that the non-image areas hold the hydrophilic properties enough to be free from stains even on printing a large number of prints.
It is known that these performance properties of the printing plate precursors are influenced by the ratio of zinc oxide to resin binder in the photoconductive layer. For example, as the ratio of resin binder to zinc oxide particles becomes small, oil-desensitization of the surface of the photoconductive layer is increased, to reduce background stains, but, in turn, the internal cohesion of the photoconductive layer per se is weakened, resulting in reduction of printing durability due to insufficient mechanical strength. On the other hand, as the proportion of the resin binder increases, printing durability is improved, but background staining tends to become conspicuous. With respect to background staining, while it is a phenomenon associated with the degree of oil-desensitization achieved, it has been elucidated that the oil-desensitization of the photoconductive layer surface depends not only on the zinc oxide/resin binder ratio in the photoconductive layer, but also greatly on the kind of the resin binder used.
Resin binders which have been conventionally known include silicon resins (see JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese Patent application"), styrene-butadiene resins (see JP-B-35-1950), alkyd resins, maleic acid resins, polyamides (see JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-11219, 36-8510, and 41-13946), etc. However, electrophotographic light-sensitive materials using these known resins suffer from several disadvantages, such as (1) low charging characteristics of the photoconductive layer; (2) poor quality of a reproduced image, particularly dot reproducibility or resolving power; (3) low sensitivity to exposure; (4) insufficient oil-desensitization attained by oil-desensitization for use as an offset master, which results in background stains on prints when used for offset printing; (5) insufficient film strength of the light-sensitive layer, which causes release of the light-sensitive layer during offset printing, failing to obtain a large number of prints; (6) susceptibility of image quality to influences of environment at the time of electrophotographic image formation, such as high temperatures and high humidities; and the like.
For the particular use as an offset printing plate precursor, formation of background stains due to insufficient oil-desensitization presents a serious problem. In order to solve this problem, various resins have been proposed as binders for zinc oxide which would improve oil-desensitization, including a resin having a molecular weight of from 1.8.times.10.sup.4 to 1.times.10.sup.5 and a glass transition point of from 10 to 80.degree. C. which is obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit having a substituent having a carboxylic group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application); a tetra- or pentamer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and 57-202544; a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group as disclosed in JP-A-58-68046; and the like.
Nevertheless, evaluations of these resins proposed for improving oil-desensitization revealed that none of them is fully satisfactory in terms of stain resistance and printing durability.
The use of resins having a functional group capable of forming a hydrophilic group on decomposition as a binder has been studied. For example, there have been proposed resins having a functional group capable of forming a hydroxyl group on decomposition, as disclosed in JP-A-62-195684, 62-210475, and 62-210476, and resins having a functional group capable of forming a carboxyl group on decomposition, as disclosed in JP-A-62-21269.
These functional group-containing resins form a hydrophilic group upon being hydrolyzed or hydrogenolyzed with an oil-desensitizing solution or dampening water used during printing. It has been reported that use of these resins as a binder of a lithographic printing plate precursor can avoid various problems associated with use of resins containing a hydrophilic group from the first, such as deterioration of surface smoothness and electrophotographic characteristics, which seem ascribable to the strong interaction between the hydrophilic group and the surface of the photoconductive zinc oxide particles. It has also been expected that the hydrophilic properties of the non-image areas attained by an oil-desensitizing solution can be enhanced by the hydrophilic group formed by decomposition of the resin, so that a clear distinction can be made between the lipophilic image area and the hydrophilic non-image area. Adhesion of a printing ink onto the non-image areas during printing can thus be prevented, thereby making it possible to obtain a large number of prints having a clear image free from background stains.
However, the above-described functional group-containing resins capable of forming a hydrophilic group are still unsatisfactory in resistance to background stain and printing durability. In particular, it has turned out that the resin becomes water-soluble as its amount is increased for further improving hydrophilic properties of the non-image areas, thus impairing durability of the hydrophilic properties. Hence, there is a demand to develop a technique by which the hydrophilic properties of the non-image areas can be assured, while still lasting long. Namely, it has been keenly desired to establish a technique in which the effect to improve hydrophilic properties can be retained or enhanced even if the proportion of the resin containing a hydrophilic group-forming functional group in the total resin binder is decreased, or a large number of clear prints can be obtained without suffering from background stains even if the printing conditions are made more strict due to an increase of a printing machine in size or a variation of printing pressure.